It's their land, their choice what to do with it.

I maintain the ones on my property since I use it, some neighbors might, and I'm fine with the public using it. Someone started making their own bike trail through a sensative area, so I made those signs go away (the orange flags they put up) and they took the hint to use the established trail a short distance away. Other than that we put up "no littering" signs and people stopped tossing dog poop bags and trash so people are generally good.

Not necessarily. For most recreational mountain bike trails I'm familiar with, access is at the landowner's discretion. Famously, a few years ago, landowners in East Burke revoked access on what had been some of the most popular trails in the state.

On the other hand, I know of some properties that the state sold to timber companies where public hiking access was a condition of the sale. Not sure what sort of legal process would need to happen to get that changed.

And then there are town highways and legal trails. These are public rights of way found on the town highway map that pass through private parcels. If a new landowner wanted to prevent access, they'd need to go through the town selectboard who would bring it to a vote. This seems to happen regularly.

They can post it, unless there's some other unusual easement or covenant that is at issue.

In addition to what others have said, it should be noted that organizations like the GMC (hiking), VAST (snow machines), and VMBA (mountain bikes) do work with landowners to be good stewards of the land.

They have to post it yearly

Same with snowmobile trails. It’s up to the landowner.

I know people that got fed up with needy hikers and kicked the Long Trail off of their property. It really messed up the LT.

The owners were so fed up with strangers asking for favors. Hikers were constantly knocking on their door.